what does it take to swap a big block in a duster ?

Hey there I have been working on my 1970 ply duster that I have set up for a pro touring style three link and watts link set up, I am also in the stages of building a fab 9 for it, I have a few ?s though my engine of choice will either be a 440 or 383 bb set up for boost, question number one is are these motors identical from the outside for the most part ? Question 2 is there any modification that has to be done to the fire wall and tranny tunnel to fiit a bb and 4 speed trans in an original 225 slant six three. Speed on the floor car ?I will be building custom floors, fire wall and tunnel just need to know wether I can build it closer to my stock pieces, if mods are required ill have to go find a block and trans for mock up but if not I should be good, thanks a lot guys